Abstract: To study the influence mechanism of spherical-cap gaps on ultrasonic detection of the arch bridge equipped with concrete-filled steel tubes and to evaluate its ultrasonic compactness after the filling is finished, proposed is a quantitative evaluation model of spherical-cap gap defects of concrete-filled steel tubes based on ultrasonic pulse velocity. A total of 1365 groups of detection data for ultrasonic pulse velocity in CFST were collected to supplement the existing database. The influence mechanism of spherical-cap gaps on ultrasonic pulse velocity were revealed through test data analysis. A calculation model of ultrasonic pulse velocity considering the effect of spherical-cap gaps was established. Considering the coupling effect of time-varying characteristics on ultrasonic pulse velocity, a quantitative evaluation model was proposed by introducing the gap ratio and the time factor according to the parameters used in the two-stage regression analysis. The applicability of the proposed model was verified by comparing with the traditional model and experimental data. Analysis shows that the existence of spherical-cap gaps (gap ratio from 0.5% to 9.2%) leads to a decrease of 4.4% to 22.8% in the ultrasonic pulse velocity detection results of concrete-filled steel tube specimens. When the spherical-cap gaps are less than 5.5%, the spherical-cap gaps defects on the ultrasonic wave velocity had a more significant impact. By combining the modified function of gap ratio and time-varying function, the proposed model not only can explicit expressive function between gap ratio and ultrasonic pulse velocity, but also be requirements of different detection ages. When the parameters of concrete compressive strength, of detection age, and of gap ratio are assigned with different values, the accuracy of the model proposed can exceed 90%, adding higher computational accuracy and applicability to the evaluation of the compacting performance of the concrete in the pipe of the arch bridge.